1. Field of the Invention
This invention relates to vacuum and gas-filled valve devices in which electrons are emitted from a cold cathode by virtue of a field emission process.
2. Description of Related Art
Over the past thirty years, semiconductor device technology has replaced vacuum valve technology for all but the most specialized electronic applications. There are many reasons for the preference for semiconductor devices. For example, they have a higher operating speed than vacuum devices, they are more reliable, they are considerably smaller and they are cheaper to produce. Furthermore, their power dissipation is much lower, particularly when compared with thermionic vacuum devices which require a considerable amount of cathode heating power.
However, it has become apparent that at least in one respect vacuum valve devices are greatly superior to devices based on solid state materials. The vacuum devices are far less affected by exposure to extreme or hostile conditions, such as high and low temperatures. Because the band gaps of useful semiconductors are necessarily of the order of lev and many other interband excitations are lower than this, excitation of intrinsic carriers occurs at temperatures only slightly above room temperature. This severely modifies the characteristics and the performance of semiconductor devices. In addition, the electron occupancy of the traps and other defect states which determine the properties of semiconductor structures is extremely temperature sensitive. The problems become increasingly acute with the trend towards smaller semiconductor devices and higher integration density.
Vacuum devices, on the other hand, suffer to a much smaller extent from such problems. The density of the conduction electrons which are responsible for thermionic and field emission processes is not dependent on temperature, and because the devices have barriers with large work functions, thermal activation requires a temperature of at least 1000.degree. K. Furthermore, it is now recognized that the most important of the previously-accepted advantages of semiconductor devices, namely their integrability and their cheapness of manufacture, derive largely from the small size of the devices rather than from their solid state nature. Hence, if vacuum devices were made in a micron size range, such devices could be insensitive to environment, whilst being as small and fast as current semiconductor devices. Indeed, it is possible that such vacuum devices could be made to operate even faster than semiconductor devices, since the ultimate speed of the electrons in vacuo would be the speed of light, whereas that in a semiconductor device is limited to a considerably lower value by scattering or by phonon emission.
Although some recent work has been done on thermionic devices, it is likely that field emission devices will prove more successful, because the field emission effect is less dependent upon temperature.
We have previously proposed a method of forming a vacuum device in which cathode, grid and anode structures are formed on a substrate, such that the structures are coplanar and the electron flow is substantially parallel to the substrate. The fabrication of such a device is simple to achieve, but the device suffers from the disadvantage that the electron path is long, which may result in a loss of operational efficiency in the device. Furthermore, large-scale integration of such devices is limited, because only a relatively low packing density can be achieved due to the flat electrode configuration.